The man who may have been bitten by a shark as he body-surfed with his teenage son off the coast of Ballston Beach in Truro, Mass., is undergoing surgery today but is expected to live, officials said.

The attack happened around 3:30 p.m. Monday when Chris Myers, a father of two, was pulled under the water by what is believed to be a shark. Police say Myers was bit in both legs below the knees in possibly one single, crushing blow.

"They [rescuers] dragged him out and they had to carry him up here and they had to wrap it around his legs and he was bleeding through the gauzes," a witness told local ABC affiliate WCVB. "It was bad. Both [of] his legs were pretty bad."

Shark experts said they must examine Myers's wounds before confirming if his injuries were caused by a shark or another marine animal. City officials said they would keep Ballston Beach, part of the Cape Cod National Seashore, open in the meantime but post shark warnings as a precaution for beachgoers.

Myers was conscious and alert when authorities loaded him into an ambulance. He was taken in stable condition to Cape Cod Hospital in Hyannis and then transferred to Massachusetts General Hospital in Boston, according to WCVB.

"It was pretty deep. You could see muscle and bone," Truro police officer Scott Holway said. "It was like his flesh had been ripped."

Several witness say they saw a shark fin appear before the attack. Witnesses watched helplessly from the beach as Myers was pulled under the water. Myers soon surfaced and screamed for help.

"Suddenly something came up between them. It had a large dorsal fin," Anne-Marie Corner, a Truro summer resident, told WCVB.

Sharks have been more visible along Cape Cod this summer. Experts say that's because of a drastic increase of seals in the area.

"The elbow of the cape has these large, dense concentrations of gray seals now, and these white sharks go to the area to feed," said Greg Skomal, a senior biologist at the Massachusetts Division of Marine Fisheries. "Because the seals are so abundant, now the white sharks are paying more attention."

A great white shark was spotted trailing a kayaker three weeks ago at Nauset Beach, about 25 miles south of Monday's attack. That's a total of four shark sightings this summer off the coast of Cape Cod.

"It's a little alarming, I have to say," a beachgoer said. "With kids and boys that are pretty brave to go far out deep in the water, it's a little alarming especially with the seals coming so close."